I don't think you are grasping how these keep alives are designed and what the trade-offs are. First, these keep alives are made with super capacitors. Super capacitors have very high storage capacity, in 1F, 2.5F, and 5 Farad values (not microfarads like a typical electrolytic capacitor is rated for but rather full farad values). However, these super capacitors used in the keep alives do not have high voltage ratings. Many have a voltage rating of only around 5.x volts. Now.. you can put these in series to raise the overall voltage; however, the overall capacitance value goes down. Capacitors in series are calculated as 1/C = 1/C + 1/C + 1/C .... You put them in parallel to add capacitance but then the rated voltage stays the same.
Here is a sample using three 2.5 Farad 5-volt capacitors in series in order to get a 15-volt super capacitor:
Note: The physical size of each of these capacitors are 3/4" x 3/4" x 3/8" (and now stack 3 of these together for 15-volt value - that is huge to cram into a loco shell or tender body).
1/C = 1/2.5F + 1/2.5F + 1/2.5F
1/C = 0.4 + 0.4 + 0.4
1/C = 1.2
C = 0.8333 Farads @15 volts
Therefore, you can see why there are none of these types of stacked super capacitor circuits for O scale out there. You only get 0.8333 Farads of capacitance which won't even give you 0.2 seconds of power even though you use three huge super capacitors. When they are bundled together, they are huge, and you don't even get the equivalent of what is available in a stock ESU Loksound decoder. I think they have around a 1 Farad onboard super capacitor in ESU Loksound V5 L.
Therefore, in order to get longer run times with higher capacitance values and maintain a manageable package size, then circuits with voltage boosters like the ones provided in the link of my previous post above (repeated again below) are used. This link has designs and parts lists for both ESU Loksound 3-wire models and 2-wire, Soundtraxx and MTH PS3 versions but uses voltage booster circuits to boost and reduce voltages at the right places. According to the test data in the link, you can get up to 6 seconds of runtime with the bigger 5 Farad capacitors.
Keepalives, O-Sized | O Gauge Railroading On Line Forum (ogaugerr.com)
The resistor (56 ohms 0.5 watt) used in the above circuit is to limit the charge rate of the capacitor. If you have too low of a resistor value or no resistor at all, then the charge rate of the capacitor will appear as a short to the DCC booster and trip the DCC circuit breaker due to the high inrush current, especially when the capacitor is fully discharged. If the resistance is too high, then it will take too long to charge the capacitor after a discharge occurs during a power interruption. Therefore, a happy medium needs to be selected.
The zener diode acts like voltage regulator. Think of zener diode as dam holding back water to regulate the water level in front of the dam at the absolute maximum (pretend in preparation for a drought). However, like a dam, it has a finite limit. When the water level exceeds the height of the dam, then the water flows over the dam, so theoretically the water level will never be higher than the height of dam (by any appreciable amount). In the case of the zener diode, if the voltage exceeds the rated value of the zener, it flows backwards through diode to ground (e.g. or over the dam) and essentially wasted energy. If the capacitor charge voltage is much higher than the zener rating, you waste a lot of power. Think of a heavy rainstorm and lots of water flowing over the dam. Therefore, it is important to rate your zener value safely below the super capacitor limit to protect the capacitor from an overvoltage condition but also not so low that you waste a lot of power when the capacitor is fully charged, and you are just dumping excess voltage (current - which could be used to run your trains) to ground in a steady state condition backwards through the zener. In the dam example, you would always want a just little water going over the dam so you know that the water level is regulated at the very top of the dam at all times.
The regular diode in the Soundtraxx current keeper designs is used as a one-way valve to allow current to flow out of the capacitor to power the decoder in the event of a power interruption but not allow power to flow backwards into the capacitor when power is restored. We want to limit the inrush current to not trip the DCC circuit breaker, so we want the capacitor charging to take place through the separate 56-ohm 0.5 watt resistor path as noted above and not through the path that the capacitor discharges.
Therefore, go back re-read the thread in the link above and see if becomes clearer why this type of circuit is used in O scale rather than just stacking a bunch of super capacitors in series and adding a resistor and some diodes that would only yield an inferior keep alive in the end.
Scott